1. Field of the Invention
The invention relates to antennas and, more particularly, to a planar antenna with a planar, inner antenna element, which is surrounded by an outer antenna element, where the inner antenna element and the outer antenna element each have a feed point.
2. Description of the Related Art
EP 1 437 792 B1 discloses a planar antenna that forms part of a cavity slot antenna for automobiles. The inner antenna element has a hexagonal shape. It is surrounded by a square loop, which acts as a grounding conductor.
EP 1 513 224 A1 discloses another planar antenna. This planar antenna has an antenna element with an approximately square basic shape. Here, however, two opposite corners of the square basic shape have been milled away. Consequently, the two diagonals, which are orthogonal to one another, of the approximately square basic shape have different lengths. An annular grounding face is also provided. This grounding face opens up in its interior to a square area, into which the antenna element is inserted. Here, the antenna element assumes a uniform distance from the inner edge of the annular grounding face, as far as the regions of the two defined corners. Finally, connection contact points for the antenna element and the grounding face are provided. These connection contact points are placed opposite one another at two side edges of the antenna element and the grounding face outside the diagonal.
U.S. Pat. No. 6,914,573 B1 discloses a small, planar antenna with a large bandwidth. This antenna has a grounding face with a rectangular outer contour which is symmetrical with respect to an axis. An approximately oval free area is located in the interior of the grounding face, and a likewise approximately oval antenna area is inserted symmetrically into the free face. The antenna area includes a connecting line that is passed to the outside through the grounding area by way of a gap lying on the axis of symmetry. Here, the width of the free area does not decrease, when viewed from the connecting line to a point opposite the connecting line. However, the width of the free area is tapered in the two symmetrical regions in the direction towards the connecting line to achieve a uniform impedance transition.
A coplanar antenna structure, which is as broadband as possible, is required for various applications, such as for use in Ultra High Frequency-Radio Frequency Identification (UHF-RFID) transponders. Read and write devices for UHF transponders in the conventional frequency band of 865 MHz to 960 MHz need only operate in a specific frequency band dependent on the regulations of the country where the system is being used. Objects which are intended to be identified by transponders are often used across borders. Therefore, transponders with a detection range in a large frequency band are of importance. Large detection ranges are only possible when the antenna system of a transponder can supply sufficient energy to the RFID semiconductor component.